Scanning probe devices such as the scanning Probe microscope (“SPM”) or atomic force microscope (“AFM”) can be used to obtain an image or other information indicative of the features of a wide range of materials with molecular and even atomic level resolution. In addition, AFMs and SPMs are capable of measuring forces accurately at the piconewton to micronewton range, in a measurement mode known as a force-distance curve or force curve. As the demand for resolution has increased, requiring the measurement of decreasingly smaller forces free of noise artifacts, the old generations of these devices are made obsolete. A demand for faster results, requiring decreasingly smaller cantilevers, only reinforces this obsolescence. The preferable approach is a new device that addresses the central issue of measuring small forces with minimal noise, while providing an array of modules optimizing the performance of the device when using small cantilevers or when doing specialized applications such as optical techniques for biology, nanoindentation and electrochemistry.
For the sake of convenience, the current description focuses on systems and techniques that may be realized in particular embodiments of scanning probe devices, the SPM or the AFM. Scanning probe devices also include such instruments as 3D molecular force probe instruments, scanning tunneling microscopes (“STMs”), high-resolution profilometers (including mechanical stylus profilometers), surface modification instruments, nanoindenters, chemical/biological sensing probes, instruments for electrical measurements and micro-actuated devices. The systems and techniques described herein may be realized in such other scanning probe devices.
A SPM or AFM is a device which obtains topographical information (and/or other sample characteristics) while scanning (e.g., rastering) a sharp tip on the end of a probe relative to the surface of the sample. The information and characteristics are obtained by detecting changes in the deflection or oscillation of the probe (e.g., by detecting small changes in amplitude, deflection, phase, frequency, etc., and using feedback to return the system to a reference state). By scanning the tip relative to the sample, a map of the sample topography or other characteristics may be obtained.
Changes in the deflection or oscillation of the probe are typically detected by an optical lever arrangement whereby a light beam is directed onto the side of the probe opposite the tip. The beam reflected from the probe illuminates a position sensitive detector (“PSD”). As the deflection or oscillation of the probe changes, the position of the reflected spot on the PSD also changes, causing a change in the output from the PSD. Changes in the deflection or oscillation of the probe are typically made to trigger a change in the vertical position of the base of the probe relative to the sample (referred to herein as a change in the Z position, where Z is generally orthogonal to the X/Y plane defined by the sample), in order to maintain the deflection or oscillation at a constant pre-set value. It is this feedback that is typically used to generate a SPM or AFM image.
SPMs or AFMs can be operated in a number of different sample characterization modes, including contact modes where the tip of the probe is in constant contact with the sample surface, and AC modes where the tip makes no contact or only intermittent contact with the surface.
Actuators are commonly used in SPMs and AFMs, for example to raster the probe or to change the position of the base of the probe relative to the sample surface. The purpose of actuators is to provide relative movement between different parts of the SPM or AFM; for example, between the tip of the probe and the sample. For different purposes and different results, it may be useful to actuate the sample or the tip or some combination of both. Sensors are also commonly used in SPMs and AFMs. They are used to detect movement, position, or other attributes of various components of the SPM or AFM, including movement created by actuators.
For the purposes of this specification, unless otherwise indicated, the term “actuator” refers to a broad array of devices that convert input signals into physical motion, including piezo activated flexures, piezo tubes, piezo stacks, blocks, bimorphs, unimorphs, linear motors, electrostrictive actuators, electrostatic motors, capacitive motors, voice coil actuators and magnetostrictive actuators, and the term “sensor” or “position sensor” refers to a device that converts a physical quantity such as displacement, velocity or acceleration into one or more signals such as an electrical signal, including optical deflection detectors (including those referred to above as a PSD and those described in U.S. Pat. No. 6,612,160, Apparatus and Method for Isolating and Measuring Movement in Metrology Apparatus), capacitive sensors, inductive sensors (including eddy current sensors), differential transformers (such as those described in U.S. Pat. No. 7,038,443 and continuations thereof, Linear Variable Differential Transformers for High Precision Position Measurements, U.S. Pat. No. 8,269,485 and continuations thereof, Linear Variable Differential Transformer with Digital Electronics, and U.S. Pat. No. 8,502,525, and continuations thereof, Integrated Micro-Actuator and Linear Variable Differential Transformers for High Precision Position Measurements, which are hereby incorporated by reference in their entirety), variable reluctance, optical interferometry, strain gages, piezo sensors, magnetostrictive and electrostrictive sensors.
Some current SPM/AFMs can take images up to 100 um2, but are typically used in the 1-10 um2 regime. Such images typically require from four to ten minutes to acquire. Efforts are currently being made to move toward what is sometimes called “video rate” imaging. Typically those who use this term include producing images at the rate of one per second all the way to true video rate at the rate of 30 per second. Video rate imaging would enable imaging moving samples, imaging more ephemeral events and simply completing imaging on a more timely basis. One important means for moving toward video rate imaging is to decrease the mass of the probe, thereby achieving a higher resonant frequency and as well a lower spring constant.
Conventional SPM/AFM probes are currently 50-450 μm in length with spring constants of 0.01-200 N/m and fundamental resonant frequencies (fR) of 10-500 kHz. Physical laws put lower limits on the achievable resolution and scan speed of conventional probes, given acceptable noise levels.
To get the best resolution measurements, one wants the tip of the probe to exert only a low force on the sample. In biology, for example, one often deals with samples that are so soft that forces above 10 pN can modify or damage the sample. This also holds true for high resolution measurements on hard samples such as inorganic crystals, since higher forces have the effect of pushing the tip into the sample, increasing the interaction area and thus lowering the resolution. For a given deflection of the probe, the force increases with the spring constant (k) of the probe. When operating in air in AC modes where the tip makes only intermittent contact with the sample surface, spring constants below 30 N/m are desirable. For general operation in fluid, very small spring constants (less then about 1.0 N/m) are desirable.
To get measurements with higher scan speeds, one wants probes with a high fR. After passing over a sample feature, the probe response is about 1/fR seconds for contact modes and Q/fR seconds for AC modes (where Q is the quality factor for the probe). This sets a fundamental limit on scanning speed: raising the response time of the probe requires a probe with a high fR or, in the case of AC modes, a low Q.
A higher fR also means lower noise operation. The thermal noise of a probe involves fixed noise energy of order kT (where k is the Boltzmann constant and T is the temperature in Kelvin) spread over a frequency range up to approximately the fR. Thus, the higher fR, the lower the noise per unit band width below fR.
Probes with a high resonant frequency and a low spring constant can be achieved by making them smaller and thinner. However, using current SPMs/AFMs with probes significantly smaller than conventional ones presents difficulties. In general, optimal optical lever detection requires that the spot from the light beam directed onto the side of the probe opposite the tip should substantially fill the area available in one dimension. Underfilling results in a loss of optical lever detection efficiency because the reflected beam diverges more than necessary. Overfilling the lever means losing light and producing unwanted interference fringes due to light reflected off the sample.
One ideal probe for video rate imaging would have a fR in the 5-10 MHz range and a force constant in the 1-40 N/m range. This implies shrinking conventional probes by an order of magnitude, to approximately 5-8 μm in length or width. Such a shrinking, taken together with the requirement that the spot substantially fill the probe, means that the spot on the probe also must be shrunk. The optical system producing the beam incident on the probe should have a numerical aperture (NA) sufficient with the wavelength of the light from the light source to form a focused spot approximately 5-8 μm in diameter in at least one direction.
The relatively large numerical aperture required to so shrink the spot results in a shallow depth of focus. This can present problems with the refocusing necessary when replacing one probe with another or when using a probe with more than one cantilever. In addition, the large opening angle of the incident beam used to achieve a high numerical aperture can require complex lens systems or an accumulation of lenses in close proximity to the probe.
A SPM/AFM that takes advantage of these smaller, high fR, high bandwidth probes is described in U.S. Pat. No. 8,370,906, Modular Atomic Force Microscope. The Cypher AFM manufactured by the assignee of that patent, as well as any patent resulting from the current application, provides a portion of the results forthcoming from these cantilevers without their actual employment. With this instrument lower noise measurements and increased imaging rates are possible without the use of smaller, high fR, high bandwidth cantilevers. The Cypher AFM routinely images point lattice defects in crystal surfaces in liquid environments.
In many applications the old generations of SPM/AFMs required the probe and sample to be relatively isolated in a local, user-controlled environment. Where the user was seeking an understanding of sample properties in a particular environment, for example in a particular liquid or particular gas, the sample and the probe used to sense the sample both had to be isolated and maintained at the environment of interest. The same was true where the user was seeking an understanding of sample properties at a particular temperature. In either case the environment so created also had to facilitate a compliant connection between the sample and the probe so that when the sample moved relative to the probe, or vice versa, the motion was minimally distorted and the image and measurements also minimally distorted.
The requirement that an understanding of sample properties in a particular environment or at a particular temperature means that the sample and the probe both have to be isolated and maintained at the environment or temperature of interest is of even greater importance when the when the user is employing smaller, high fR, high bandwidth probes or is using a SPM/AFM like that described in U.S. Pat. No. 8,370,906, Modular Atomic Force Microscope, (which includes the Cypher AFM manufactured by the assignee of that patent). In order to fully achieve the resolution and imaging rates made possible by these probes and SPM/AFMs when a particular environment or particular temperature is important, isolation is even more critical than with old generations of SPM/AFMs.
FIG. 1 shows a cross section of a prior art apparatus for sealing the probe and sample. In this design an o-ring 4020 or other flexible seal seals the volume around the probe 1040 and between the cantilever holder 4010 and the sample 1030 mounted on the scanner 4000. The compliant nature of the o-ring 4020 produces relatively undistorted motion between a moving sample scanner and static cantilever holder; or between a static sample scanner and a moving cantilever holder.
However several performance issues arise with the apparatus shown in FIG. 1 The most significant is the relatively small diameter of typical sealing elastomeric o-rings (˜1 mm) severely constrains our ability to design an apparatus that eliminates distortion in the scanning motion. These o-rings simply are not compliant enough. The FIG. 1 design is notorious for distorting the scanning motion of relatively weak open loop tube scanners. However, even with stiffer scanners employing piezoelectric stacks and closed loop sensors, the FIG. 1 design will cause scanning motion distortion as the load dependent elasticity of the o-ring deflects the mechanical structure between the sample and X/Y sensors (not shown) housed in the sample scanner 4000. This is especially obvious when the user is acquiring a series of relatively small images (scan of <100 nm) separated by relatively large (>5 um) offsets. As the o-ring relaxes after an offset, the relaxing force exerted by the o-ring 4020 on the mechanical structure between the sample and the sample scanner 4000 causes creep in the subsequently acquired image, the more so the less time is allowed between scans.
Given the interest in observing dynamic phenomena, the cell design should incorporate ports that allow for liquid and/or gas perfusion thereby allowing the cell environment to be changed during imaging or other measurements. The port positioning is important for ensuring complete exchange of fluid during perfusion experiments. Additionally, the cell should be able to maintain moderate pressures (˜5 psig) thereby allowing gravity flow perfusion. Gravity forced perfusion is a simple, yet noise free method for flowing liquids during AFM measurements.
Temperature dependent effects in materials are of extreme importance. As devices begin to shrink further into the sub-100 nm range following the trend predicted by Moore's law, the topic of thermal properties and transport in such nanoscale devices becomes increasingly important. In addition, basic material science requires in depth understanding of the nanoscale thermodynamics of materials. Polymer crystallization for example, determines in great extent the macroscopic mechanical properties of the material but is mediated by nanoscale effects.
Temperature control on nanoscale devices while they are being measured is also of great importance. Temperature differences between the measurement point and the thermometry can cause significant errors in the quantification of, for example, various thermodynamic transitions including the melting and glass transitions in polymers.
FIG. 2 shows a cross section of a prior art heater for maintaining the probe and sample at or near a particular temperature. These heaters used a sample block 1000 made of a material with a high thermal conductivity, such as copper. A heating element 1010 was included within or attached to the block 1000 as was a temperature measuring means 1020. The temperature measuring means 1020 was of course used to measure the temperature of the sample block 1000 and might also be used in a control circuit (not shown). The sample 1030 was mounted to the top surface of the sample block 1000 and the sample block was in turn mounted on a scanner (not shown). The probe 1040 was positioned above the sample.
One important challenge posed by the FIG. 2 heater is that the temperature measuring means 1020 is extremely difficult to place next to the region of the sample 1045 that is being imaged or measured by the tip of the probe 1040. Temperature gradients tend to make the temperature of this region 1045 different from the temperature measured by the temperature measuring means 1020.
A second challenge is minimizing image drift. To this end, materials with low thermal expansion must be used in the mechanical structure between the sample and X/Y sensors (not shown) housed in the sample scanner (not shown) on which the sample block 1000 is mounted.
A third challenge is managing the extraction of excess heat. In heating applications it is critical to maximize the thermal resistance between the sample heater and the elements of the mechanical structure between the sample and X/Y sensors (not shown) which may expand/contract with temperature changes and lead to degraded imaging performance. At an extreme, if the temperature of the Z-axis actuator rises above its Curie temperature, it will lose its actuation ability and the microscope will be rendered inoperable. This same problem affects other SPM/AFMs covered by prior art and has been solved by inserting a liquid cooled metal block between the piezoelectric actuator and the heat source. Sufficiently thin and flexible rubber hoses connect this block to a mechanical pump which circulates cooling water. Sufficiently pliable hoses will minimize scan distortion but often the mechanical vibrations of the pump and the pulsation of the fluid flow will introduce undesirable noise and deteriorate instrument performance. Fluid leaks which damage the instrument are also not uncommon.
A similar problem arises in cooling applications where thermoelectric devices are an attractive and compact method for cooling the sample. With these devices the minimum temperature reachable on the cold side of the device depends heavily on the efficiency of heat extraction from the hot side. Active methods of extracting heat from the hot side of the device include using pumped coolant. Although pumped coolant is an efficient method for heat extractions it complicates the design with the addition of pumps and fluid routing in a very constrained space. Additionally, pumps can add an unacceptable amount of acoustic and vibration noise to the SPM/AFM measurements.